1. Field of the Invention
The present invention relates to a method for fabricating an insulated gate type semiconductor device such as a thin film transistor.
2. Description of Related Art
Because an active matrix liquid crystal display is small, light and able to display high resolution motion pictures at high speed, it is expected to become one of the most used displays of the future. However, because the substrate of a liquid crystal display needs to be translucent, the type of substrate to be used with the display is limited. A glass substrate, a quartz substrate or a plastic substrate may be cited as examples thereof However, a plastic substrate has a problem in that it lacks heat resistance and a quartz substrate has a problem in that it is very expensive. Its cost is more than 10 times that of a glass substrate especially when it is widened, although it can withstand a high temperature of around 1000.degree. C. Accordingly, the glass substrate is widely used in general for the reasons of heat resistance and economy.
Currently, the performance required of liquid crystal displays is increasing and the performance and characteristics required of a thin film transistor (hereinafter referred to as a TFT) used as a switching element in such displays is also increasing. Due to this, research for forming a crystal silicon film on a glass substrate is being actively conducted. The crystal silicon film is formed on the glass substrate by forming an amorphous silicon film and crystallizing it by heat treatment at the present.
Because the heat resistance temperature of the glass substrate is normally about 600.degree. C., a process which exceeds this temperature cannot be used for forming the crystal silicon film. Therefore, a method of forming amorphous silicon film by means of plasma CVD or reduced pressure CVD and of crystallizing it by heat has been adopted in forming a crystal silicon film on a glass substrate.
A technology of crystallizing the silicon film by irradiating laser light is also known and allows a crystal silicon film having an excellent crystallinity to be formed on the glass substrate has an advantage that the laser light will not damage the glass substrate thermally.
However, the crystal silicon film crystallized from the amorphous silicon film has had a large number of defects caused by dangling bonds and the like. Because these defects are a factor causing degradation of the characteristics of the TFT, it is necessary to passivate the defects at the interface between the active layer and the gate insulating film and the defects within and at the boundaries of the crystal grains of the silicon of the active layer when fabricating a TFT utilizing such crystal silicon films. The defects at the grain boundaries in particular are the main factor causing scattering of charge. However, it is very difficult to passivate the defects at the grain boundary.
It is possible to compensate the defects at the grain boundaries of a crystal silicon film by Si when fabricating a TFT on a quartz substrate because it is possible to implement a heat treatment at a high temperature of around 1000.degree. C. In contrast to this, it is difficult to implement a heat treatment at high temperatures when fabricating the TFT on a glass substrate. Thus, the defects at the grain boundaries of the crystal silicon film are passivated by hydrogen by implementing a hydrogen plasma treatment in an atmosphere of about 300 to 400.degree. C. normally in the final stage of the process. An n-channel type TFT has a practical field-effect mobility when the hydrogen plasma treatment is implemented.
On the other hand, the effect of the hydrogen plasma treatment is not so remarkable for a p-channel type TFT. This is assumed to happen because a level caused by the defect of the crystal is formed in a relatively shallow domain under a conduction electron zone.
Although it is possible to compensate the defect of the grain boundaries of the crystal silicon film by implementing the hydrogen plasma treatment, the reliability of the TFT or the n-channel type TFT, in particular, which has been treated by the hydrogen plasma, is not stable because the hydrogen compensating the defect is apt to be desorbed. For instance, if the n-channel type TFT is energized for 48 hours in an atmosphere of 90.degree. C., its mobility is reduced to a half.
Further, although the quality of the crystal silicon film obtained by irradiating laser light is good, ridges (irregularity) are formed on the surface of the crystal silicon film if the thickness of the film is less than 1000 angstrom. When laser light is irradiated to the silicon film, the silicon film melts instantly and expands locally. The ridges are formed on the surface of the crystal silicon film to relax internal stress caused by this expansion. The difference of elevation of this ridge is about 1/2 to 1 times the thickness of the film. For instance, when laser annealing is implemented after crystallizing an amorphous silicon film whose thickness is about 700 .ANG. by way of heating, ridges of 100 to 300 .ANG. in height are formed on the surface thereof.
Because a potential barrier and a trap level caused by the dangling bonds, distortion of the lattice and the like are formed at the ridges on the surface of the crystal silicon film in an insulated gate type semiconductor device. Hence, the level of the interface between the active layer and the gate insulating film becomes high. Further, because the peak of the ridges is sharp and thus an electric field is apt to concentrate there, it may become a source of leakage current, eventually causing dielectric breakdown. Further, because the ridges on the surface of the crystal silicon film lessen the coating quality of the gate insulating film deposited by way of sputtering or CVD, the reliability of the insulation is degraded thus resulting in defective insulation.